The present invention relates to methods for the isolation of biological polymers from plant material comprising parenchymal cells, especially from sugar beet, citrus and related pulps. More particularly, this invention deals with the isolation of cellulose from sugar beet and citrus pulp and other materials, which cellulose is found to have unique structural, chemical, physical and rheological properties. This invention is also concerned with the isolation of non-cellulosic biopolymers, including hemicelluloses, from such materials, and with the production of novel vegetable gums. This invention is further directed to the simultaneous, economical isolation and recovery of cellulosic and noncellulosic biopolymers from parenchymal cell-containing material, especially sugar beet and citrus pulp.
Spent sugar beet pulp, a by-product of the sugar beet processing industry, is comprised predominantly of three biopolymers, pectin, arabinogalactan and cellulose. Other naturally occurring biological constituents of sugar beets such as fats, proteins, soluble oligosaccharides, and other low molecular weight components, are largely extracted from sugar beets during the removal of sucrose therefrom. The remaining polysaccharides in sugar beet pulp comprise generally conjugated, particulate cell residuals having morphologies generally characteristic of parenchymal cells found in certain higher plants.
Typically, the solid composition of sugar beet pulp comprises about 40% cellulose, about 30% arabinogalactan, and about 30% pectin. Minor amounts of protein, tannins and residual, low molecular weight carbohydrates are also generally present. The gross composition varies slightly with species, growing conditions and time of harvest. Historically, few economical uses have been found for spent sugar beet pulp. It is a material which spoils rapidly and consequently constitutes a local environmental problem. In contrast to the solid residue obtained from the processing of sugar cane, sugar beet pulp has a negative fuel value. Accordingly, it takes more energy to dehydrate sugar beet pulp to a combustible state than can be recovered from its burning. Sugar beet pulp has been admixed with molasses and dried for use as ruminate feed, however, alternative markets for molasses have largely led to the discontinuation of its addition to pulp. The low intrinsic nitrogen content of molasses-free pulp reduces its feed value to simple roughage. Moreover, the energy costs attendant to the drying of sugar beet pulp render the resulting material so expensive that it is becoming uncompetitive even as a feed extender. Similar considerations apply to raw citrus pulp which cannot be dried efficiently and which, accordingly, is almost always limed to facilitate mechanical dewatering.
From time to time, it has been proposed to extract certain of the hemicellulosic components of sugar beet, citrus and other pulp for commercial use. Thus, the hydrolysis of sugar beet pulp in either acidic or basic conditions together with the isolation and hydrolytic modification of hemicelluloses, especially pectin, therefrom has been reported. In all cases where high thermal and/or acid or base assisted extractions were performed upon sugar beet pulp, it was perceived that increasing temperature and acid or base strength led to undesirable hydrolytic degradation of the arabinogalactan and/or pectin components of the pulp. The prior art has, accordingly, indicated the inadvisability of the use of high temperatures and strongly acidic or basic conditions for the hydrolytic processing of sugar beet pulp to obtain hemicelluloses.
The cellulosic components of sugar beet and citrus pulp have not heretofore found significant commercial use. Such cellulose has always been viewed as possessing no exceptional physical or chemical qualities. Accordingly, the isolation of cellulose derived from sugar beets and other parenchymal cell sources has not been the object of significant study apart from the traditional areas of paper making, formulation of cellulosic chemicals, and the like. Accordingly, it has not been known to isolate parenchymal cell cellulose from sugar beet pulp or from other parenchymal cell-containing plants.
It has not been known to provide processes which permit the simultaneous, sequential isolation of both cellulosic and hemicellulosic constituents of sugar beet, citrus and other parenchymous pulp in high yield with a low degree of degradation of the hemicellulose. In prior attempts to isolate one or more cellulosic or hemicellulosic materials from sugar beet and other such pulps, it has not been known to treat such pulp under conditions which favor such simultaneous isolation of the important biopolymers of the pulp.
U.S. Pat. No. 4,025,356 issued to Nyman et al. discloses a method for the continuous hydrolysis of pentose-containing materials. According to this process, pentosans such as furfural, are recovered from bagasse, straw, chaff, reeds, corn husks, corn cobs, wood chips and similar materials. Lengthy hydrolysis (4-5 hours) at temperatures between 80.degree. and 120.degree. C. at pH's lower than 5 are disclosed. A secondary hydrolysis at higher temperatures (between 140.degree. and 180.degree. C.) to facilitate the isolation of furfural is also disclosed.
U.S. Pat. No. 4,018,620 to Penque discloses the isolation of monosaccharides from cellulose through hydrolysis with low cnncentrations of acid. Hydrolysis with calcium-chloride and inorganic acid yields, for example, glucose from cotton fibers, or alpha cellulose.
U.S. Pat. No. 4,029,515 issued to Kiminiki et al. discloses a process for the decrystallization of cellulose through the mixing thereof with phosphoric acid followed by extraction with aqueous tetrahydrofuran at room temperature.
U.S. Pat. No. 4,160,695 issued to Dietrichs et al. discloses the production of glucose from cellulose-containing vegetable matter through treatment with saturated steam at a temperature from about 160.degree. to 230.degree. C. for a period of from 2 minutes to 4 hours. Subsequent treatment with alkali liberates the products. It is also disclosed to treat vegetable matter containing xylans with steam at a temperature of from about 180.degree. to 220.degree. C. for periods of time from 5 minutes to 60 minutes. Liberation of xylan hemicellulose may be accomplished thereby.
U.S. Pat. No. 4,174,976 issued to Tsao et al. discloses the acid hydrolysis of cellulose to obtain glucose. Certain co-reactants are added to improve the degradation of the crystalline cellulose.
U. S. Pat. No. 4,281,063 to Tsao et al. isolates cellulose from cellulosic materials through acid or base treatments to remove the hemicellulose followed by solvent extraction of the residue to dissolve the cellulose and by reprecipitation thereof. The reprecipitated cellulose is hydrolyzed either with acid or by enzymes. The initial hydrolysis is acid catalyzed with from about 0.5 to 5% sulfuric acid at about 90.degree. to 140.degree. C. for from 15 to 300 minutes.
U.S. Pat. No. 4,266,981 issued to Tsao et al. discloses a new step in the acid hydrolysis of cellulosic materials. The first stage hydrolyzes cellulose with dilute acid to remove the hemicellulose portion to yield a liquid hydrolysate containing pentose sugar. Hydrolysis of the solid residue with a small amount of concentrated sulfuric acid dissolves and partially hydrolyzes the remaining cellulose which may be then separated from lignin, reprecipitated and subsequently hydrolyzed to glucose.
U.S. Pat. No. 4,023,982 issued to Knauth discloses the hydrolysis of hemicellulosic materials with acid and steam to isolate the constituent sugars. Isolation of the hemicelluloses is not disclosed.
Loeb et al. in "Preparation of Cotton Cellulose IV from Cotton Cellulose III", J. Poly. Sci., Vol. IXV, No. 73 (1954), reports that cellulose III may be transformed into cellulose IV through dissolution in glycerine for 64 hours followed by heating for 3 hours at 250.degree. C. under nitrogen. The structure of cellulose IV was said to be "considerably degraded by the high temperature glycerin treatment . . . ".
U.S. Pat. Nos. 3,212,932 issued to Hess et al., and 4,237,226 issued to Grethlein, also refer to the hydrolysis of cellulosic vegetable material to recover constituent mono saccharides.
U.S. Pat. No. 4,201,596 issued to Church et al. discloses the continuous saccharification of cellulosic materials through high temperature acid hydrolysis. Use of strong mineral acid and steam at elevated temperatures for short time periods such as from under 1 to over 5 minutes results, according to Church, in the depolymerization of cellulose into its constituent sugars.
Isolation of hemicellulosic materials from plant waste has also been the subject of patents and publications. Thus, U.S. Pat. No. 4,160,695 discloses the non-acidic treatment of vegetable matter with steam at temperatures of from about 180.degree. to 220.degree. C. for from 5 minutes to 60 minutes to liberate xylan hemicellulose. U.S. Pat. No. 4,239,906 issued to Antrim et al. discloses the isolation of cellulose and hemicelluloses from corn hulls. "Relatively mild conditions", a pH from 2.2 to 5.5, at temperatures from 70.degree. to 100.degree. C. for from about 30 to 60 minutes are employed to liberate hemicellulose fractions from cellulosic residue. Antrim teaches that more vigorous hydrolysis such as hydrolysis at pH levels below about 1 for longer periods at higher temperatures are suitable for the isolation of cellulose but cause substantial degradation of the hemicellulose fraction.
Numerous patents issued to Battista including exemplary U.S. Pat. Nos. 3,141,875 and 2,978,446 are drawn to the production of microcrystalline cellulose. Such cellulose is not predominantly parenchymal in origin and may not be obtained concomitantly with the isolation of hemicellulose.
U.K. Patent No. 2,066,145 issued to I.T.T. discloses a cellulose denominated "microfibrillated cellulose". Such material is created by shearing a liquid suspension of cellulose derived from wood pulp.
Other U.S. patents disclose treatment of cellulosic materials with differing combinations of pH, temperature and time. These include U.S. Pat. Nos. 4,070,232 issued to Funk, 3,479,248 issued to Nobile, 4,168,988 issued to Riehm et al., 2,803,567 issed to Owens and British patent No. 555,842 issued to McDowell. Certain literature surveys are of interest including "Extraction of Hemicelluloses from Plant Materials" by Yanobsky, Industrial and Engineering Chemistry, January 1939, pp. 95 et.seq.; and Miller and Savage, "By-Products from Sugar Beets", Chemurgic Digest, April 1948. None of the foregoing references are believed to suggest the critical combination of pH, time, temperature, and mechanical shear taught by the present invention which allows the isolation of cellulosic and hemicellulosic components of sugar beet pulp without substantial degradation thereof. None of the foregoing discloses the novel cellulosic products which may be obtained from parenchymal cell-containing material through the practice of the present invention. Moreover, none of the foregoing disclose or suggest novel gums which may be obtained from the practice of certain embodiments of this invention.